1. Field of the Invention
The present invention relates to a method for manufacturing, by form rolling, a multi-grooved V pulley having a plurality of V grooves, for configuring a transmission unit with a poly V belt (multi-grooved V belt) having a plurality of V-shaped ridges, and a preliminary form roller used for the method.
2. Description of the Related Art
A conventional method of manufacturing a multi-grooved V pulley by form rolling will be explained with reference to FIGS. 1a to 1c. In a preliminary forming process, an annular material 1 having, in a sectional view a peripheral edge as shown in FIG. 1a, is held in a rotary die having a plurality of parts in a forming apparatus not shown and is rotated at a predetermined rotational speed. At the same time, as shown in FIG. 1b, a first form roller 2 having a flat peripheral edge surface (having a linear outer peripheral surface in a sectional view) is pressed radially against a channel-shaped groove 3 in the peripheral edge of the material 1. By thus rotating the material 1 and the roller 2 together, the channel-shaped groove 3 is forced wide open to thereby fabricate an annular intermediate material 5 having a wide V groove 4 with a flat bottom (linear in a sectional view).
Then, in the finish form rolling process shown in FIG. 1c, a finish form roller 6 with a peripheral edge having a sectional shape similar to that of the surface of the poly V belt is pressed against the intermediate material 5, and by thus rotating the material and the roller together, a multi-grooved V pulley 8 is roll formed as a product having a plurality of V grooves 7 corresponding to a plurality of V-shaped protrusions 15 of the finish form roller 6.
In this finish form rolling process, the V-shaped protrusions 15 of the finish form roller 6 bite into the intermediate material 5 while rotating thereby to form a plurality of V grooves 7 of the multi-grooved V pulley 8. At the same time, a plurality of ridges 9 are formed. The manner in which each corrugation ridge 9 is formed on the intermediate material 5 is shown sequentially in FIG. 2.
First, in stage a, the intermediate material 5 held in a rotary die not shown is designated by 5a. The finish form roller 6 shown in FIG. 1c is pressed against the flat bottom surface of the wide V grooves 4, whereby the intermediate material 5a is deformed into a material 5b shown in stage b. The material 5b thus deformed develops annular recesses 10 at the portions pressed by the V-shaped protrusions 15 of the finish form roller 6. At the same time, annular bulges 11 are formed between the two adjoining recesses 10 by the flow of molecules of the metal material. The bulges 11, however, are not flat and the portions thereof near the recesses 10 are comparatively high. Thus, a depression 12 is formed between a pair of the recesses 10.
In stage c of FIG. 2 in which the finish form roller 6 shown in FIG. 1c further bites into the intermediate material 5b, a sharp annular edge 13 is formed at each of the two corners of the deformed intermediate material 5c. Each edge 13 has a hardness increased by work hardening of the metal, and depending on the material, the hardness may reach the order of HRB 93 for a steel material. In addition, the depression 12 becomes deeper.
When the finish form roller 6 further bites into the intermediate material 5c into stage d, the bulges 11 rise higher under the pressure exerted by the side surfaces of the V-shaped protrusions 15 of the finish form roller 6 into a shape almost identical with the shape of the V-shaped ridges 9 of the final multi-grooved V pulley 8. Although the depression 12 converges at the crests of the bulges 11, the left and right edges 13 are overlapped and remain unremoved as an annular crack 14. The defect like the annular crack 14 formed at the forward end of the crest of the V-shaped protrusion 9 is called a "top roll".
The left and right edges 13 which are overlapped and constitute the annular crack 14 as a top roll never completely coalesce with each other, if left as they are. Even in the final stage e, therefore, an irregular annular crack 14 remains at the forward end of the crest of the ridge 9 over the entire periphery of the multi-grooved V pulley 8. Therefore, the ridge line of the annular V-shaped protrusion 9 of the multi-grooved V pulley is not smooth but in a rough state. In addition, the hardness at the forward end of the crest is increased by the work hardening of the material. In the case where a transmission unit is configured by winding a poly V belt of rubber or like on the multi-grooved V pulley 8 thus fabricated, the poly V belt may be damaged by the crack 14 in the ridge line of the V-shaped protrusion 9 of the multi-grooved V pulley 8, thus shortening the life of the poly V belt. Further, the edges 13 of high hardness develop during the form rolling process as described above. Therefore, the side surfaces of the V-shaped protrusion 15 of the finish form roller 6 are cut off, and the period (service life) during which it can be used as a tool is shortened.
A method of manufacturing a multi-grooved v pulley conceived for solving the problem of the top roll is described in JP-A-60-216942. In this improved conventional method, as shown in FIGS. 3a to 3c, a preliminary form roller having, in a sectional view, a comparatively low corrugated peripheral edge not shown is pressed against the bottom surface of the channel-shaped groove 3 of the annular material 1 shown in FIG. 3a similar to the one shown in FIG. 1a. In this way, a preliminary form 17 is fabricated which has a plurality of comparatively low ridges 19 formed on the bottom of the wide V groove 16 as shown in FIG. 3b.
In the next step, the finish form roller not shown is pressed against the preliminary form 17, thereby roll forming the multi-grooved V pulley 18 as shown in FIG. 2(c). The finish form roller used in the process includes a plurality of V-shaped protrusions corresponding to a plurality of the V grooves 7 of the multi-grooved V pulley 18. The V-shaped protrusions have the same intervals as the wavelength of the corrugation of the preliminary form roller or the wavelength of the ridges 19 of the corrugation of the preliminary form 17, and are higher than the wave crest of the preliminary form roller.
According to the improved conventional method described above, the part of the bottom of the V groove 16 of the preliminary form 17 corresponding to the intermediate portion formed by the V-shaped protrusions of the finish form roller does not constitute a depression 12 as shown in FIGS. 2(b) or (c), but is preformed as corrugation ridges 19. When the preliminary form 17 of FIG. 3b is processed into the multi-grooved V pulley 18 of FIG. 3c, therefore, the forward end of the crest of each of the V-shaped ridges 20 is prevented from developing a top roll including the annular crack 14 as shown in FIG. 2.